1. Field of the Invention
The present disclosure relates to the control of circuit breakers, and more particularly, to the control of circuit breakers in a multi-phase impedance grounded or un-grounded low voltage power system during concurrent faults.
2. Description of the Related Art
Three-phase power systems are used in many industrial processes. A three-phase power system is one in which currents for three sinusoidal voltages are carried through a power distribution device, and in which the sinusoidal voltages are 120 degrees out of phase with one another. The three-phase power system, similar to a single-phase power, uses circuit breakers to interrupt the flow of current through a power line in the event of a ground fault. A ground fault is a situation where single-phase current that flows through a circuit protective device to the load, returns to a source through a circuit path back to a current source, without flowing through the same device or circuit through which the current is intended to flow in both directions.
In a solidly grounded system single-phase power, such as is found in a typical residential home or most commercial installations, if there is a ground fault in a circuit a circuit breaker trips (i.e., opens), hence there is no power conveyed to the circuit until the breaker is reset (closed). However, in many three-phase applications, where interruption to the power is highly undesirable, an impedance grounded or an ungrounded system may be employed. In such a system a “first ground fault” creates very little fault current. This creates a situation where the ground fault may be allowed to persist and power continues to flow to the load un-interrupted. This may be the case, for example, in a chemical plant where removal of power during a chemical process can have serious, and indeed catastrophic, consequences. Typically, therefore, a limiting resistance, e.g., 100 ohms, is interposed in a ground path to the current source to limit the first ground fault current to a few amperes, typically 5 or less.
However, there is a possibility of a concurrent occurrence of a first fault in a first phase of a three-phase system, and a second fault in a second phase of the three-phase system. This sets up a circuit with phase to phase voltage differential between the two poles involved, this is then the basis for a phase-to-phase fault current flowing, through ground, between the first and second phases. Consequently, the aforementioned grounding resistance is bypassed. The phase-to-phase fault current is much greater than the fault current in the case of a single ground fault. The phase-to-phase fault current can present a serious hazard, and as such, should be interrupted immediately.
However, in the case of concurrent faults, opening a circuit breaker to interrupt the phase-to-phase current may pose a problem. When a current path is suddenly opened, the protective device must clear an arc formed by the current through the gap created by the opening of the device's contacts. This produces significant physical stress on the protective device.
One type of protective device commonly used are circuit breakers. There are two types of circuit breakers commonly used. Low Voltage Power Circuit Breakers UL listed under UL standard 1066, and Molded Case Circuit Breaker UL listed under standard UL 489. These devices differ in operating capabilities. One difference is that molded case circuit breakers are designed to employ two or more poles within the same circuit breaker to interrupt fault current up to the listed capability of the device. The ability of these devices to interrupt maximum potential single phase fault current with a single pole at rated voltage may be extremely limited. The UL 489 standard does not require UL489 circuit breaker to have significant single pole capability. It is not expected that these circuit breakers are to be used in systems that are impedance grounded, where short circuit currents may be large and ground faults may be prevalent. The second kind of circuit breaker is built to UL 1066 and it does have substantial single pole ratings. Specifically, 87% of the short circuit rating determined for the circuit breaker while clearing full 3-phase fault. Low Voltage Power Circuit Breakers are may be larger and significantly more expensive than electrically equivalent circuit breakers built to UL 489 standards.
Therefore, there is a need for a safer technique of controlling circuit breakers in a case of a concurrent fault on two or more phases of an impedance grounded or un-grounded multi-phase power system.